1. Field of the Invention
The present invention relates to an optical device and optical unit transmissive of light, and to an imager capable of generating an image signal through focusing the light of from a subject.
2. Description of the Related Art
Recently, it is popular to incorporate an imager, capable of acquiring a digital photographic image by taking a picture of a subject, in a small-sized appliance such as a cellular phone or a PDA (personal digital assistant). By providing an imager on a small-sized appliance carried at all times, shooting is available at any time in a manner freely from carrying a digital or video camera in the related art.
Meanwhile, the endoscope is usually used at the medical site, to insert a small-sized imager in the body cavity and take images of the body interior. It is a recent practice to excise, at the site, a lump, etc. found upon photographing, by use of the excision tool attached on the endoscope. The endoscope draws an increasing demands as a medical device relieved of the burden on the human body.
It is the recent tendency toward using an imager considerably smaller in size as compared to the usual digital camera, as described above. The photographic image, as taken by the small-sized imager, is under improvement in quality by virtue of the development of small-sized CCDs high in pixel density and small-sized lenses improved in contrast. As for the problems left, there is a strong desire to mount a small-sized imager with such auto-focus and zoom functions as mounted as standard on the usual digital camera.
Auto focus and zoom functions are generally realized by moving a plurality of lenses in a direction along the optical axis (hereinafter, the direction along the optical axis is referred to as a front-rear direction) in the imager. However, in the small-sized imager for use in an endoscope, there is a difficulty in moving the lens within the imager or arranging a massive motor or cam mechanism to drive the lens.
In relation to this, there is proposed a liquid lens that the focal length can be changed by changing a liquid-surface form under the application of a voltage to an electrically-conductive liquid (see JP-A-2005-62864 and “Philips' Fluid Lenses, [online], Mar. 3, 2004, Royal Philips Electronics, [searched on Mar. 31, 2004], the Internet <URL: http://www.dpreview.com/news/0403/04030302philipsfluidlens.as p>, for example). For example, “Philips' Fluid Lenses, [online], Mar. 3, 2004, Royal Philips Electronics, [searched on Mar. 31, 2004], the Internet <URL: http://www.dpreview.com/news/0403/04030302philipsfluidlens.as p> describes a liquid lens structured with a tube having an inner wall covered with a water-repellent coating, a conductive water-based liquid and non-conductive oil-based liquid sealed in the tube, and an electrode for applying an electric field to the water-based liquid in the tube. In the liquid lens, the water-based liquid is a semispherical solid in the state no voltage is applied to the conductive water-based liquid, so that the water-based liquid and the oil have an interface assuming a convex form. The interfaces changes from the convex to a concave depending upon the magnitude of an electric field applied to the conductive water-based liquid. For this reason, radius-of-curvature varies as a lens thus enabling to change the focal length without restriction.
With such a liquid lens, focal length can be changed without moving the lens. Without providing a motor or cam mechanism as mentioned before, zoom and focus functions can be realized. Accordingly, the apparatus can be greatly reduced in size, and hence applied to a small-sized appliance, such as a cellular phone or an endoscope.
However, according to the liquid lens described in JP-A-2005-62864 or “Philips' Fluid Lenses, [online], Mar. 3, 2004, Royal Philips Electronics, [searched on Mar. 31, 2004], the Internet <URL: http://www.dpreview.com/news/0403/04030302philipsfluidlens.as p>, the conductive solution might be electrolyzed by the charge released from the electrode. Where used over a long term, the produced gas builds up into bubbles within the liquid container thus resulting in a problem that light scatter takes place to lower the transmittance of light.
This problem is generally true for the optical elements, such as parallel plates and prisms, without limited to the lens.